The present invention relates to the manufacture of substrates for multiple wavelength laser arrays. More particularly, the invention provides a technique including a method and device for the manufacture of multiple-wavelength vertical-cavity laser arrays by using patterned-substrate molecular beam epitaxy (MBE). Specifically, the present invention is directed toward forming a multiple wavelength laser array employing a MBE process that includes, the thermal redesorption. In other embodiments, an in-situ monitoring procedure is employed.
Vertical-cavity surface-emitting lasers (VCSELs) offer many advantages for applications in optical communications, optical interconnects, optical neural networks and optical signal processing. In the early stages of development, VCSELs were single wavelength devices. To that end, early VCSELs were typically formed by a process developed by Jewell et al. which is described in U.S. Pat. No. 4,949,350. In the process of Jewell et al., a vertical planar semiconductor structure is epitaxially grown. The semiconductor structure consists of upper and lower distributed Bragg reflectors separated by an optical distance equal to the lasing wavelength, defining a resonant cavity. An active quantum well layer is positioned in the middle of the cavity. After growth of the planar structure, an array of lasers are formed thereon by an etching process. Electrical current is passed through the selected lasers of the array causing emission of light. The surface-normal topology of VCSELs facilitates the integration of these devices as one-or two-dimensional arrays. This feature, combined with the inherent single Fabry-Perot mode operation, makes VCSELs desirable structures for wavelength division multiplexing (WDM) applications where ultrahigh bandwidth and various functionalities are required. One such application for WDM is discussed by Buckman et al. in "A novel all-optical self-routed wavelength-addressable network (SWANET)," IEEE Photon. Technol. Lett., vol. 7, no. 9, p. 1066-8, 1995 and describes the use of VCSELs as simultaneous and reconfigurable interconnects. In an article by Willner et al., "2-D WDM optical interconnects using multiple-wavelength VCSEL's for simultaneous and reconfigurable communication among many planes," IEEE Photon. Technol. Lett., vol. 5, pp. 838-841, 1993, VCSELs are described as being employed to facilitate wavelength routing. L. F. Stokes in "Towards wavelength division multiplexing," IEEE Circuits and Devices Magazine, vol. 12, no. 1, pp. 28-31, 1996 describes one advantage of using multiple-wavelength VCSEL arrays with wide channel spacings (.about.10 nm) to provide an inexpensive solution to increasing the capacity of local area networks without using active wavelength controls.
The demand for multiple wavelength laser arrays led to the development of a monolithic multiple-wavelength VCSELs, described by Chang-Hasnain in U.S. Pat. No. 5,029,176. The monolithic multiple-wavelength VCSELs described by Chang-Hasnain are in either a one or two dimensional arrays and are formed on a common substrate by sequential vertical growth of a lower reflector, a lower spacer, an active light emitting region and upper spacers and reflectors.
Resonant cavities are grown between the upper and lower reflectors. The optical lengths of the resonant cavities determine the lasing wavelengths. To achieve wavelength emission at multiple wavelengths, non-uniform growth of the structure over part of the substrate which defines the resonant cavity is performed. To obtain non-uniform deposition , either thermal gradients or mesas and valleys of different widths are present in the substrate.
There has been extensive efforts in controlling the parameters of resonant wavelength cavities of VCSELs. C. J. Chang-Hasnain et al. in "Multiple wavelength tunable surface-emitting laser arrays," IEEE J. Quantum Electron., vol. 27, pp. 1368-1376, 1991 discuss implementation of the intrinsic beam nonuniformity in an MBE system as being suited for controlling formation of a resonant cavity.
Control of the resonant cavity parameters has also been achieved by varying local GaAs growth rate by creating a temperature gradient on the surface of a substrate brought about by the presence of a thermally conductive pattern disposed on a side of the substrate positioned opposite to the side on which GaAs growth occurs, L. E. Eng et al., "Multiple wavelength vertical cavity laser arrays on patterned substrates," IEEE J. Select. Topics in Quantum Electron., vol. 1, pp. 624-628, 1995; W. Yuen, et al., "Multiple-wavelength vertical-cavity surface-emitting laser arrays with a record wavelength span," IEEE Photon. Technol. Lett., vol. 8, pp. 4-6, 1996; and W. Yuen, et al., "Location-resolvable optical monitored growth of multiple-wavelength vertical-cavity laser arrays," Electron. Lett., vol. 31, pp. 1840-1842, 1995.
F. Koyama et al., in "Two-dimensional multi wavelength surface emitting laser arrays fabricated by nonplanar MOCVD," Electron. Lett., vol. 30, pp. 1947-1948, 1994, describes a nonplanar MOCVD growth technique. Post-growth processing to control the resonant cavity parameters has been advocated by T. Wipiejewski et al. in "Vertical cavity surface emitting laser diodes with post-growth wavelength adjustment," IEEE Photon. Technol. Lett., vol. 7, pp. 727-729, 1995; and T. Wipiejewski et al. in "Multiple wavelength vertical-cavity laser array employing molecular beam epitaxy regrowth" Electron. Lett., vol. 32, no. 4, p. 340-342. 1996.
Still another approach for controlling the resonant cavity parameters of VCSELs involves shadow-masked MBE growth, see H. Saito, I. Ogura, Y. Sugimoto, and K. Kasahara, "Monolithic integration of multiple wavelength vertical-cavity surface-emitting lasers by mask molecular beam epitaxy," Appl. Phys. Lett., vol. 66, pp. 2466-2468, 1995; and H. Saito, I. Ogura, and Y. Sugimoto, "Uniform CW operation of multiple-wavelength vertical-cavity surface -emitting lasers fabricated by mask molecular beam epitaxy." IEEE Photon. Technol. Lett., vol. 8, no. 9, p. 1118-20, 1996. Among all these approaches, however, precise control of the wavelength selective region of the VCSEL has been difficult to achieve.
What is needed, therefore, is a growth technique which allows precise control of the wavelength selective region of a multi wavelength VCSEL.